Such antennas, which may be highly directional antennas for point-to-point transmission or sector antennas for point-to-multipoint transmission must often be covered by cladding plates on buildings in order to avoid a deterioration of the aspect of the building. Such cladding plates inevitably have an influence on the radiation pattern of the antenna. In order to keep this influence small, it is known e.g. from DE 199 02 511 A1 to adapt the thickness d of such a cladding plate to the vacuum wavelength λ0 of the radiation emitted by the antenna and to the dielectric constant ∈R of the plate material according to the formula
  d  =            m      2        ⁢                            λ          0                                      ɛ            R                              .      
A beam which is oriented perpendicular to the plate surface and is reflected at the exit side of the plate reaches the incidence side delayed by m wavelengths, so that it interferes, due to a phase shift π at the boundary, in phase opposition with the incident beam and thus suppresses reflection at the cladding plate.
A wave which is not incident perpendicularly on the cladding plate has to propagate in it on a longer path, so that the condition for absence of reflection is no longer fulfilled, and the transmission through the cladding plate may be attenuated considerably.
FIG. 1 illustrates this problem by means of azimuth cuts of the directivity pattern of an assembly formed of a 90° sector antenna and a cladding plate made of glass fibre-reinforced plastic which is perpendicular to a main beam direction of the sector antenna. The cut shown as a solid line exhibits a slight, tolerable angular dependency of the amplitude inside the sector and a strongly varying amplitude at low levels outside the sector. In practice, perpendicular incidence can often not be realized because the orientation of the cladding plate is in most cases predetermined by the outline of a building facade behind which the antenna is mounted, whereas the orientation of the antenna is defined by constraints such as the position of a cell to be covered by the antenna or, in case of a point-to-point connection, the position of a partner antenna, which constraints have no relation to the building. Considering the case of the main beam direction of the antenna and the surface normal of the cladding plate forming an angle of 20° with respect to each other in the horizontal plane, as represented in FIG. 1 as a dashed line, it is found that the reflection, which is now no longer suppressed completely at the cladding plate, causes a specular image of the antenna beam to appear at angles above 100°. In a practically relevant assembly in which four 90°-sector antennas located at a same place cover four radio cells which meet at the place of the antenna, this means that the radio signal of the considered antenna is radiated with a non-negligible intensity into one of the other cells and affects reception there.
FIG. 2 illustrates the problem in the elevation direction. As shown in curve E of the elevation cut, the beam is strongly directed in the horizontal direction, in order to achieve a wide range at a low transmission power. Off the horizontal plane the radiated intensity is much lower, but it must not vanish because otherwise reception would not be possible in a close range around an antenna mounted in an elevated position. The curve E of the elevation cut should therefore extend between two constraint curves R+, R−. This may be achieved with an uncladded antenna, but with a cladded antenna, the problem arises that the intensity radiated at a non-vanishing angle with respect to the horizontal plane cannot fulfil the condition for absence of reflection at the same time as the intensity radiated in the horizontal direction. Due to reflection losses, the elevation cut E of the cladded antenna drops below the constraint curve R− in some places.